In a vehicle installed with a continuously variable transmission, to ensure that slippage does not occur in a belt looped around a primary pulley and secondary pulley during rapid deceleration, for example, a primary pulley pressure is detected by a primary pulley pressure sensor, and when the primary pulley pressure decreases, a speed ratio change control valve is forcibly displaced to a speed ratio position in which oil pressure is temporarily introduced to the primary pulley. In so doing, belt slippage on the primary pulley side is suppressed. This so-called speed ratio fixing control is disclosed in JP2004-125011A.